Speaker
Description
Lepton-flavour conservation is an almost perfect symmetry in the Standard Model of particle physics, violated so far only by neutrino oscillations. Neutrino mixing implies that lepton-flavour violation occurs in charged decays as well, albeit with extremely low branching fractions ($\mathcal{B}$) of order $\mathcal{O}(10^{-50})$. Therefore, any direct observation would be a clear and unambiguous sign of new physics beyond the Standard Model. The search for the $\tau^+\to\phi(K^+K^-)\mu^+$ decay provides a sensitive probe for several new physics scenarios, including vector leptoquark models which predict branching fractions up to $10^{-8}$, close to current experimental sensitivities. This contribution presents the first-ever attempt to search for $\tau^+\to\phi(K^+K^-)\mu^+$ in hadronic collisions. The measurement is currently performed with the data collected by the LHCb experiment at CERN during Run 2 (6$\,\mathrm{fb}^{-1}$, 2015-2018). Based on the Run 2 experience and thanks to the Data Acquisition and Trigger systems of the brand-new LHCb-Upgrade I detector, LHCb is now collecting an enriched sample of $\tau^+\to\phi(K^+K^-)\mu^+$ decay candidates by means of the first dedicated exclusive trigger line implemented at the earliest stage of the High Level Trigger. The new trigger increases the signal selection efficiency by a factor of about 6 compared to Run 2, with LHCb expected to collect $\sim23\,\mathrm{fb}^{-1}$ by the end of Run 3 in 2026. The analysis strategy developed for the Run 2 dataset is presented, along with the design and performance of the new exclusive trigger selection in Run 3. Finally, the expected sensitivity on the upper limit with Run 2 and Run 3 datasets is also discussed.